High value vertical resistors by ion implantation and method for making same

ABSTRACT

A vertical resistor is described comprising aluminum, a high atomic number metal such as tungsten and implanted oxygen ions to adjust the resistivity of the aluminum to the desired level of resistance. The tungsten layer is formed intermediate an upper and lower layer of the aluminum. In the preferred embodiment, these three metallic layers can be formed by sputtering. The high atomic number material is employed to prevent the implanted ions from penetrating to the lower aluminum layer along an aluminum grain particularly adapted for ion channelling. The implanted ions assume a distribution within the upper aluminum layer beginning at a point under the surface and peaking at the intermediate high atomic number layer. This distribution of the ions determines the resistivity of the aluminum.

United States Patent Gurev [451 Aug. 5, 1975 [75] Inventor: Harold S. Gurev, Scottsdale, Ariz.

[73] Assignee: Motorola, Inc., Chicago, Ill.

[22] Filed: Feb. 28, 1974 21 Appl. No.: 446,838

[52] US. Cl. 338/308; 29/620; 357/91 [58] Field of Search 338/204, 308; 357/91; 29/620, 576 B; 148/6 [56] References Cited UNITED STATES PATENTS 3,325,258 6/1967 Fottler 338/308 X 3,472,688 10/1969 Hayashi... 338/308 X 3,591,413 7/1971 Seki 338/308 X 3,818,413 6/1974 Krimmel 338/308 X 33 OXYGEN IMPLANTED ALUMINUM Primary ExdminerE. A. Goldberg Attorney, Agent, or FirmVincent J. Rauner 5 7 ABSTRACT A vertical resistor is described comprising aluminum, a high atomic number metal such as tungsten and implanted oxygen ions to adjust the resistivity of the aluminum to the desired level of resistance. The tungsten layer is formed intermediate an upper and lower layer of the aluminum. In the preferred embodiment, these three metallic layers can be formed by sputtering. The high atomic number material is employed to prevent the implanted ions from penetrating to the lower aluminum layer along an aluminum grain particularly adapted for ion channelling. The implanted ions assume a distribution within the upper aluminum layer beginning at a point under the surface and peaking at the intermediate high atomic number layer. This distribution of the ions determines the resistivity of the aluminum.

4 Claims, 4 Drawing Figures ALUMINUM TUNGSTEN ALUMINUM SILICON PATENTEDAUE 5l975 3,898,607

OXYGEN EMPLANTED 35 33 ALUMINUM ALUMINUM TUNGSTEN ALUMINUM HIGH VALUE VERTICAL RESISTORS BY ION IMPLANTATION AND METHOD FOR MAKING SAME BACKGROUND OF THE INVENTION The prior art is replete with references describing the use of ion implantation for implanting ions at various regions beginning at the surface and extending into the body or beginning at a point below the surface of the body and extending further into the body. Accordingly, this should be accepted as part of the prior art known to the applicant.

Additionally, the prior art shows an ion profile of oxygen atoms implanted into a metal film which assumes a gaussian distribution within the metal layer. However, these teachings do not recognize the existence of aluminum atoms oriented such as to operate as a channel for the implanted oxygen atoms. Such improperly oriented aluminum grains provide a short circuit for adjacently positioned aluminum grains. This short circuiting effect destroys the operation of such a structure as a vertical resistor. Accordingly, while it is acknowledged that the prior art does show the implantation of oxygen into metal layers in an effort to achieve a vertical resistor, such a vertical resistor can be rendered inoperative due to the fact that no effort is made to compensate for the improperly oriented grains of aluminum.

SUMMARY OF THE INVENTION The present invention relates to the fabrication of vertical resistors by ion implantation and, more specifically, relates to the fabrication of high value vertical resistors by ion implantation employing a sandwich member as the resistor.

Another object of the present invention is to provide a vertical resistor comprising .a sandwiched structure wherein the sandwich comprises a high atomic number material sandwiched between low atomic number materials.

A still further object of the present invention is the use of a high atomic number material such as tungsten sandwiched between two low atomic number layers of aluminum wherein the tungsten layer acts as a barrier for the implanted ions.

Another object of the present invention is to form a high value vertical resistor using the implantation of oxygen atoms into aluminum to adjust the resistivity of the aluminum to the desired level and using a tungsten intermediate layer as a barrier to limit the maximum penetration of the implanted ions.

BRIEF DESCRIPTION OF THE INVENTION A high value vertical resistor structure and the method for making such a resistor is described. In its broadest sense, the resistor comprises a sandwich structure wherein a high atomic number metal layer is sandwiched between an upper and lower low atomic number metal layer. The preferred low atomic number metal layers are aluminum but, beryllium or other low atomic number metals can also be used. The preferred high atomic number metal layer is tungsten but, molybdenum and other high atomic number metals can be used. Oxygen is employed in the preferred embodiment as the implanted ions.

The structure of my new vertical resistor overcomes a manufacturing problem caused by the presence of channels in aluminum. The presence of these channels are unpredictable and are caused by the random orientation of grains of aluminum. One such orientation permits implanted ions to pass essentially through the grain of aluminum to a much greater depth. This type of orientation is the exception rather than the rule. Usually the adjacently positioned grains have the standard orientation such that the implanted ions only pass partially through the grain when accelerated by the same degree of implantation energy.

A vertical resistor made in this manner such as to have a number of improperly oriented aluminum grains acting as an ion channel does not have a proper resistance value. The value of the resistor is adversely affected by short circuit paths caused by the improperly oriented aluminum grains having their high resistivity layers at a greater dpeth than the adjacently positioned grains. This creates a short circuit between adjacent grains and lowers the overall resistance value for thte resistor.

The present invention contemplates the placement of a high atomic number metal layer intermediate to an upper and lower low atomic number metal layer to effectively seal off such ion channels. Aluminum is selected for use because a lower aluminum layer assures a good ohmic contact between the vertical resistor and the semiconductor body. The upper aluminum member assures a good ohmic contact between the vertical resistor and the remaining aluminum metallization pattern on the upper surface of the semiconductor device. Therefore, the high atomic number intermediate layer seals the ion channels between the upper and the lower aluminum layers. This results in a layer of aluminum having a uniform and predetermined resistivity.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1 and 2 are cross section views of the prior art useful for understanding the present invention and,

FIGS. 3 and 4 are similar cross section views of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, curve 1 shows the standard gaussian distribution of oxygen ions implanted into an aluminum metal grain A having a standard orientation for an acceleration of 40,000 (40 KEV) electron volts. Curve 2 shows the gaussian distribution of oxygen ions implanted into an aluminum metal grain B having the improper grain orientation for the same value of acceleration energy. The grain B provides an ion channel such that the ion penetrates deeper into the grain for the same level of acceleration energy. Referring to FIG. 2, a silicon substrate is shown at 5 having an aluminum layer 7 formed on an upper surface 8 of the substrate. The interface 9 between the layer 7 and substrate 5 should provide a good ohmic contact between the two. A vertical resistor is generally indicated at 11 having a plurality of properly oriented grains A and a single improperly oriented grain B. The region 13 illustrated by the double lines 15 and 17 is the region of maximum ion accumulation and indicates the region of increased resistivity which changes the aluminum or a conductor to aluminum as a resistor. The resistive region 13 of grain B is deeper into the grain body creating a short circuit path indicated by arrows 19 and 21. The short circuit paths 19 and 21 are essentially conductive and reduce the overall resistive value to the resistor 11 to a level lower than desired.

Referring to FlG. 3, there can be seen a vertical resistor structure 23 made according to the teaching of the present invention. A silicon substrate is shown at 25 having a surface aluminum layer at 27 and generating a silicon-aluminum interface at 29. Sandwiched between an upper layer 27a and a lower layer 2712 of the aluminum layer 27 is a layer 31 of high atomic number metal such as tungsten. As was shown earlier with reference to FIG. 2, the resistor 23 comprises a majority of aluminum grains A having the proper orientation. The region of increased resistivity is shown at 33 positioned between lines 35 and 37.

The insertion of a layer of high atomic member metal at or just below the level of maximum oxygen implantation in a type B grain produces several distinct effects.

The high atomic number metal can be any one of those metals which are compatible with aluminum metallurgically, i.e., tungsten, molybdenum, platinum, palladium, tantalum, uranium and hafnium and others. Tungsten is the preferred high atomic number metal. But we will speak specifically of aluminum-tungsten here.

The aluminum outer layer deposited on top of the tungsten layer will force the nucleation of new crystals of aluminum. There will be a low degree of likelihood of a B type grain nucleating on top of B grains in the bottom layer. If the fraction of B grains in the bottom layer is X and the faction in the top layer is Y, then the faction of paths where two B-type grains are lined up in a direct line is X'Y. X'Y is less than X which is also the probability of finding a B-type grain in a single layer of aluminum film.

The tungsten itself will stop oxygen and form tungsten-oxygen which is known to have useful resistor properties when formed by chemical means.

Oxygen ions which penetrate B-type grains to the tungsten layer position and will lose substantial energy and will not channel as deeply even if they encounter a B type grain beneath the tungsten layer.

When using an intermediate tungsten layer 31, the resistivity zone 33 is positioned at a uniform distance within the upper layer 27a of the aluminum.

The worst case is shown in the left half of FIG. 4 where the oxygen ions penetrate the tungsten layer. However, the ion distribution is high at the tungsten layer. This prevents shorts from developing at the A-B grain junction.

A typical application of the ion implanted vertical resistor would be for emitter ballasts on microwave power transistors. For instance a watt, 3 GHz transistor might require lOfl/mil of emitter area for proper ballasting. If the resistor layer thickness was confined to 2000A of the metallization over the silicon contacts, the resistor layer resistance must be 3.22Qcm. to provide the proper vertical resistor ballast. If present state of the art is considered, the only material useful in this application would be doped polysilicon which has a high temperature coefficient of resistivity and is difficult to control in resistivity in this range.

While the invention has been particularly shown and described in reference to the preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is: 5 l. The method for forming a vertical resistor in combination with a semiconductor body comprising the steps of:

forming a first layer of aluminum on said semiconductor body followed by a tungsten layer and followed by an aluminum layer to form a sandwich structure, and

implanting oxygen ions at a predetermined depth into said sandwich structure.

2. A vertical resistor comprising:

a body of semiconductor material having an upper surface;

a first layer of aluminum on said upper surface;

a layer of tungsten on said first layer of aluminum;

a second layer of aluminum on said tungsten layer;

and

a layer of predetermined resistivity positioned in said first aluminum layer and formed of implanted oxygen ions for establishing the resistance value for the resistor.

3. The method for forming a vertical resistor in combination with a semiconductor body comprising the steps of:

forming a first layer of aluminum on said semiconducting body;

forming a second metal layer of a high atomic number metal selected from the group of tungsten, molybdenum, tantalum, platinum, palladium, uranium and hafnium;

forming a second aluminum layer on top of said layer of high atomic number metal to form a sandwhich structure of said first and second aluminum layers and said second metal layer; and

implanting oxygen ions into said sandwich structure to a predetermined depth.

4. A method for forming a vertical resistor comprising the steps of:

providing a semiconductor body having an upper surface;

forming a first layer of aluminum on said upper surface and said aluminum layer comprising a plurality of aluminum grains at least one of which has an orientation such as to encourage ion channelling to a depth greater than ion penetrate for the same ac- 50 celeration energy in adjacent aluminum grains;

forming a second layer of high atomic number metal of sufficient thickness such as to stop most ions which might penetrate to a depth at which the high atomic number metal layer is positioned; forming a second aluminum layer comprising a plurality of aluminum grains which nucleate on the tungsten layer having a thickness sufficient to form a vertical resistor therewithin to form a sandwhich structure of said first and second aluminum layers and said second metal layer; and

implanting oxygen ions into said sandwich structure to a predetermined average depth above said tungsten layer. 

1. THE METHOD FOR FORMING A VERTICAL RESISTOR IN COMBINATION WITH A SEMICONDUCTOR BODY COMPRISING THE STEPS OF: FORMING A FIRST LAYER SANWHICH ALUMINUM ON SAID SEMICONDUCTOR BODY FOLLOWED BY A TUNGSTEN LAYER AND FOLLOWED BY AN ALUMINUM LAYER TO FORM A SANDWICH STRUCTURE, AND INPLANTING OXYGEN IONS AT A PREDETERMINED DEPTH INTO SAID SANDWICH STRUCTURE.
 2. A vertical resistor comprising: a body of semiconductor material having an upper surface; a first layer of aluminum on said upper surface; a layer of tungsten on said first layer of aluminum; a second layer of aluminum on said tungsten layer; and a layer of predetermined resistivity positioned in said first aluminum layer and formed of implanted oxygen ions for estAblishing the resistance value for the resistor.
 3. The method for forming a vertical resistor in combination with a semiconductor body comprising the steps of: forming a first layer of aluminum on said semiconducting body; forming a second metal layer of a high atomic number metal selected from the group of tungsten, molybdenum, tantalum, platinum, palladium, uranium and hafnium; forming a second aluminum layer on top of said layer of high atomic number metal to form a sandwhich structure of said first and second aluminum layers and said second metal layer; and implanting oxygen ions into said sandwich structure to a predetermined depth.
 4. A method for forming a vertical resistor comprising the steps of: providing a semiconductor body having an upper surface; forming a first layer of aluminum on said upper surface and said aluminum layer comprising a plurality of aluminum grains at least one of which has an orientation such as to encourage ion channelling to a depth greater than ion penetrate for the same acceleration energy in adjacent aluminum grains; forming a second layer of high atomic number metal of sufficient thickness such as to stop most ions which might penetrate to a depth at which the high atomic number metal layer is positioned; forming a second aluminum layer comprising a plurality of aluminum grains which nucleate on the tungsten layer having a thickness sufficient to form a vertical resistor therewithin to form a sandwhich structure of said first and second aluminum layers and said second metal layer; and implanting oxygen ions into said sandwich structure to a predetermined average depth above said tungsten layer. 